1. Field of the Invention
The invention relates to a cyanine dye which combines the functions of IR-dye and contrast dye in a lithographic printing plate precursor and to lithographic printing plates comprising such dyes.
2. Description of the Related Art
In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, typically paper. In offset lithographic printing the ink is first transferred to a rubber blanket and then to paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image consisting of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing only ink is supplied to the master.
In the so-called “direct-to-plate” method, a digital document is transferred directly to a printing plate precursor by means of a platesetter. A printing plate precursor used in such method is often called a digital plate. With the so-called thermal digital plates, an infrared laser is used to image infrared radiation sensitive precursors. There are different types of thermal digital plates, the most important ones being: negative working plates comprising thermoplastic polymer particles of which the imaging mechanism is based on coalescence of those polymer particles; negative working photopolymerizable plates of which the imaging mechanism is based on polymerization of the images areas and positive working plates of which the imaging mechanism is based on photosolubilization.
These infrared radiation sensitive precursors have as common ingredient a compound that triggers the imaging mechanism by absorbing and converting the infrared radiation, which is used to image the precursors, into heat. Such compounds are often dyes, commonly referred to as IR-dyes.
Another ingredient all these plates have usually in common is a contrast dye and/or pigment. Such a contrast dye or pigment absorbs in the visible region and, due its presence in the print areas of the plate after exposure and processing, gives rise to an image, which can be visually inspected before printing.
It has been observed that such IR-dyes and/or contrast dyes, present as major ingredients in the coating of the precursors, can also have an impact on properties of the precursor other than the ones described above. For example it is well known that in positive thermal plates, of which the image forming mechanism is based on rendering the exposed areas of the coating more soluble in a developer, the IR-dyes as well as the contrast dyes may function as inhibitors, i.e. rendering the coating less soluble in a developer. Also, in EP-A 1914069 it was found that in a lithographic printing plate precursor comprising thermoplastic polymer particles, an IR-dye and a contrast dye, the total amount of IR-dye and contrast dye may have an influence on the clean-out behaviour of the precursor. It may be that an optimized total amount of IR-dye and contrast dye adsorbed on the surface of the polymer particles renders those particles more dispersible in an aqueous solution (for example the developer) and therefore results in an improved clean-out.
When the lithographic properties of a precursor are influenced by the total concentration of IR-dye and contrast dye, it has been observed that these properties may change upon ageing of the precursor. For example in case of a printing plate precursor comprising thermoplastic polymer particles, an IR-dye and a contrast dye, it has been observed that upon ageing of the precursor, especially at high relative humidity, the clean-out behaviour may become insufficient. Such an insufficient clean-out may then result in toning on the printed sheet. It may be that, upon ageing of the precursor, especially at high relative humidity, the IR-dyes and/or contrast dyes adsorbed on the surface of the polymer particles undergo a redistribution, resulting in a worsening of the clean-out behaviour. Especially precursors of which the grained and anodized aluminium support has not been subjected to a so-called post anodic treatment may be subject to such a worsening of the clean-out behaviour upon ageing.